Loan agreement

Hi, My sister had taken money from some investors in order to invest the same in shares and give them good returns.

Due to the share market collapse and lack of returns she was unable to give returns.The investors had forced her to sign an agreeement under duress wherein they made it look like a loan agreement on stamp paper  wherein the investment amount plus the returns she had promised was shown as the loan amount.

Please advise if such an agreement is valid and what can we do against it